The present invention relates to loop sensing apparatus for traffic detection.
Such loops are well known and used commonly for monitoring traffic flow along the lanes of roadways. Typically a loop may comprise a rectangular outline loop of conductor buried just beneath the surface of the roadway and connected to energising and detecting equipment at the side of the roadway. The loop is energised with alternating current at a selected frequency to produce a corresponding alternating magnetic field in the space above the loop. Vehicles passing over the loop affect the inductance or another parameter of the loop and this can be detected by the detection equipment. Typical prior art loops comprise a single rectangular winding having a length, in the distance of travel of vehicles along the roadway, which may be a substantial proportion of the length of vehicles travelling along the roadway, say 1 meter or more, and a width transversely in the direction of travel only slightly less than the width of the roadway lane. The detection signal produced in such loops responds to the metal mass of a vehicle passing over the loop, particularly the engine and drive train, and also chassis components of longer vehicles. For detection of vehicles as a whole, loops are designed to ensure a good detection signal is achieved as the vehicle passes by. U.S. Pat. No. 3,983,531 discloses a typical inductive loop sensor roadway installation of this kind. Loops of this kind are often referred to as inductive loops and the parameter affected is usually the inductance. However other parameters could be affected, such as the Q value of a resonant circuit incorporating the loop.
There is also a requirement to count the number of axles of vehicles passing along a roadway so that multi axle vehicles for example can be distinguished from ordinary domestic automobiles for example. Accordingly, loops have been designed which are intended to be specifically sensitive to axles, or more particularly to the wheels, of vehicles passing over the loop. U.S. Pat. No. 5,614,894 discloses a wide variety of inductive loops used for the detection of the wheels of vehicles passing along the roadway. A separate loop may be used for each wheel track in each lane of the roadway and the patent indicates that the overall length of the loops in the direction of traffic movement should be relatively short, comparable to the footprint on the roadway of the vehicle wheels to be detected by the loops.
U.S. Pat. No. 5,614,894 also discloses (in FIG. 11) an arrangement comprising an axle detecting loop located within a larger size loop. The two loops are effectively connected in series from the same length of conductor. It is stated in the specification that this loop permits the determination of the length and the speed of the vehicle, though no indication is given as to how this is achieved.
The present invention provides loop sensing apparatus for detecting vehicles travelling along a lane of a roadway, the apparatus comprising an outer loop configured to provide at least one primary surface region of the roadway lane over which magnetic field produced by current in the outer loop has the same polarity, an inner loop sized to fit and located within said primary surface region, said inner loop being configured to provide a first partial surface region of the roadway lane which is within said primary surface region and over which magnetic field produced by current in said inner loop has a first polarity and a second partial surface region of the roadway lane which is within said-primary surface region and over which magnetic field produced by the same current in said inner loop has second polarity opposite to said first polarity, and detection circuitry connected to permit each of said outer and inner loops to be energised individually and arranged to be responsive to respective detection signals generated in each of the loops by vehicles passing over the loops.
Using an inner loop having the configuration set out above, enables the outer and inner loops to be inductively decoupled so that separate detection signals can be obtained from each of the two loops. Because the inner loop has first and second partial surface regions providing magnetic field of opposite polarity for the same energising current in the inner loop, and these two partial surface regions are located in a primary surface region of the outer loop which has the same magnetic field polarity, it can be seen that a magnetic field produced by the outer loop should produce minimal EMF in the inner loop, and vice versa. The apparatus thus provides a more compact arrangement enabling individually independent detection signals to be obtained from the two loops.
Typically, the outer loop will be used for detecting the chassis of a vehicle as a whole, whereas the inner loop may be used for detecting a wheel or axle of the vehicle. Importantly, because such an axle loop detector provides its detection signal within the time frame of the detection signal from the main outer loop detecting the vehicle as a whole, it becomes much easier to ensure assignment of axle detections to the correct vehicle detection, thereby improving the reliability of systems intended to measure the number of axles of vehicles passing over the loop.
The inner loop may be configured to provide a central conducting segment and outer conducting segments spaced on opposite sides of said central segment whereby an electric current in the inner loop flows in a first direction along said central segment and in a second direction opposite to said first direction along each of said outer segments. Such a loop configuration is known as a figure-of-eight loop or a double D loop.
The central segment and one of the outer segments effectively enclose said first partial region and the central segment and the other of the outer segments enclose the second partial region. The outer segments of the inner loop may be symmetrically spaced on opposite sides of the central segment.
Preferably for detecting wheels and axles, the central and outer segments of the inner loop extend transversely to the traffic flow direction in the roadway lane. Again for wheel detection, the distance between the outer segments of the inner loop may be between 20 cms and 60 cms.
The first and second partial regions of the roadway lane provided by the inner loop, may have substantially the same area. However, if there is a substantial non uniformity in the field strength produced by current in the outer loop, this could be compensated for by adjusting the relative areas of the first and second partial regions of the inner loop, in order to maintain minimal inductive coupling between the two loops.
Preferably the outer loop has a leading edge and a trailing edge relative to the traffic flow direction in the roadway lane and the inner loop is located asymmetrically relative to a median line substantially halfway between the leading and trailing edges of the outer loop. Then, the relative timing of the detection signals from the inner and outer loops can provide an indication of the direction of travel of a vehicle over the loop. Conveniently, the inner loop can be located nearer to the leading edge of the outer loop.
The apparatus may include a second outer loop of the same form as the first mentioned outer loop and located, relative to the first outer loop, upstream in the traffic flow direction along the roadway lane, the detection circuitry then further permitting the second outer loop to be individually energised and being responsive also to detection signals generated in the second outer loop by vehicles passing over the second outer loop. Such an arrangement not only allows the direction of a vehicle over the loops to be confirmed, but also permits the correct detection of vehicles entering the detection zone in the normal traffic flow direction, coming to a stop on the loops, and then reversing back off the loops. It is especially useful to be able to detect such a manoeuvre when detection loops of this kind are used for example at the entry of a toll lane. Previous arrangements have had considerable difficulty in detecting when a vehicle entering the lane does not proceed onwards but instead reverses backwards off the detection zone.
The apparatus may also include an additional inner loop of the same form as the first mentioned inner loop and located within a said primary surface region of the outer loop at a position downstream in the traffic flow direction relative to the first inner loop, the detection circuitry then further permitting said additional inner loop to be individually energised and being responsive also to detection signals generated in the additional inner loop by vehicles passing over the additional inner loop. Such an arrangement enables a composite loop structure within the confines of a single outer loop, to be used for detecting vehicle direction, vehicle speed and also vehicle length.
A pair of the inner loops may be located side-by-side across the width of the outer loop at the same position in the traffic flow direction, each of the pair of inner loops being located within a said primary surface region, the detection circuitry then further permitting each of the inner loops to be individually energised and being responsive also to respective detection signals generated in each of the inner loops by vehicles passing over the loops. Such an arrangement enables a response to be obtained from each of the wheels on a single axle of a vehicle.
In one arrangement, the outer loop has the same form as the inner loop, being configured to provide first and second partial surface regions corresponding to the partial surface regions of the inner loop, the inner loop then being sized to fit and located within one of the first and second partial surface regions of the outer loop. Usually the central and outer segments of the outer loop are arranged to extend transverse to the traffic flow direction. Then the outer loop can be for axle/wheel detection and the inner loop enables the direction of travel across the axle detector to be identified.